Fluorescent lamps and particularly the so-called rapid-start type of fluorescent lamps are often utilized in locations where they are subjected to frequent changes in operational conditions. In other words, the fluorescent lamp is frequently turned on and off. As a result, designing a single filament which will provide both efficient and fast starting, as well as efficient and sustained operation, has typically represented compromise in an attempt to satisfy widely diverse conditions.
More specifically, starting a fluorescent lamp is best accomplished by an operating electrode which reaches operating temperature as rapidly as possible, has a low heat capacity or thermal inertia and draws enough power to produce a high operating temperature such that electron emission is effected during the rise in temperature of the electrode. On the other hand, operation of the fluorescent lamp is best accomplished by an electrode which has a relatively large electron emitting surface, a large reservoir of electron emissive material and operates at a relatively low temperature. Obviously, a single electrode or filament which attempts to fulfill both objectives is at best a compromise solution.
One known attempt to alleviate the above-mentioned undesirable compromise conditions is suggested in U.S. Pat. No. 3,215,811, entitled "Start-Run Plural Cathode Structure" assigned to the Assignee of the present application. Therein, separate starting and operating filaments are connected in parallel to a pair of electrical conductors. The starting electrode or filament is nearest the end wall of the envelope, has a light coating of emissive material and is fabricated to reach an electron emitting condition in a relatively short period of time. The operating electrode or filament further from the end wall has a heavier coating of emissive material and operates at a relatively low temperature.
Another known structure is set forth in U.S. Pat. No. 3,328,622, issued to C. L. Tommey and assigned to the Assignee of the present application. Therein, a primary electrode is coated with an electron emissive material and connected to a pair of electrical conductors formed for energizing the primary electrode. A secondary electrode surrounds the primary electrode and is also connected to a pair of electrical conductors exiting an envelope. The secondary electrode is absent electron emissive material except for that deposited thereon in response to energization of the primary electrode.
A further fluorescent lamp having a dual cathode structure is set forth in U.S. Pat. No. 3,504,218, issued to T. J. Emidy et al. Therein, a pair of electrodes is located at an angle of less than about 180 with respect to one another such that one electrode will deposit electron emissive material on the other when the one electrode is functioning. Also, one of the pair of electrodes is connected to a pair of electrical conductors while the other electrode is either short-circuited or has only one side thereof connected to an energizing source. In operation, one electrode operates until stripped of electron emissive materials, and the other electrode then becomes operational. This other electrode re-deposits electron emissive materials onto the first electrode which again is suitable for initiating conductivity of the lamp.
Although the above-mentioned dual-filament lamp sturctures have been utilized with varying degrees of success, it has been found that there are applications wherein such structures leave something to be desired, i.e., reduced lamp wattage. More specifically, it has been found that arc transfer between electrodes is dependent upon the potential differential at the emissive site of one electrode with respect to the point of arc transfer to the other electrode. For example, an AC filament voltage applied to one electrode will, depending upon the phase, exhibit a tendency not to transfer an arc to the other electrode when the voltage of the other electrode is at a lower potential. Thus, it has been found that the delay in arc transference from one electrode to the other results in a drastic reduction in operational life of the discharge lamp.